1. Field of the Invention
The present invention relates to a radar system using a circularly polarized signal. More particularly, the present invention relates to a super compact radar system of high sensitivity capable of receiving and sending a circularly polarized signal using a single circularly polarized antenna.
2. Description of the Related Art
A radar system is a system emitting a signal wave into free space using an antenna and receiving a signal wave reflected from a target object, thereby sensing a distance, a location, and so on, with respect to the target object. Such a radar system may be implemented in a large system such as a military radar or a weather radar, but recently, is being also implemented in a small system enabling individuals to use, for example, a vehicle rear-side sensor.
The radar system essentially uses an antenna to receive and send a signal wave. The antenna is classified into the linearly polarized antenna and the circularly polarized antenna according to the polarization property. The linearly polarized antenna refers to an antenna receiving and sending a linearly polarized wave (LP) traveling in a linear vector locus in a direction perpendicular to a signal-traveling direction. The circularly polarized antenna refers to an antenna receiving and sending a circularly polarized wave (CP) traveling in a circular spring-shaped locus with an electric field rotating on a vibration plane. The circularly polarized wave (CP) can be divided into a right-hand circularly polarized wave (RHCP) traveling while rotating in the right direction and a left-hand circularly polarized wave (LHCP) traveling while rotating in the left direction.
If the circularly polarized antenna is used and a transmission signal is emitted as a left-hand circularly polarized wave, the transmission signal turns into a right-hand circularly polarized wave since the characteristics of the polarized wave changes when the polarized wave is reflected from a target object. Therefore, if the left-hand circularly polarized wave antenna is used for a transmission antenna, the right-hand circularly polarized wave antenna has to be used for a reception antenna to enable a signal reflected from a target object to be received. As above, if a radar system uses the circularly polarized wave antenna, two antennas must be used which are opposite in polarization characteristics to each other. However, if two antennas are used, there exists a problem in that the radar system becomes too large in size. Therefore, it is common to use one linearly polarized wave antenna to build a small-sized radar system.
If one linearly polarized wave antenna is used, there occurs an interference problem between transmission and reflection signals. In order to prevent such a problem, a circulator or a coupler is used which can isolate the transmitter stage from the receiver stage.
Since the isolation degree between the transmitter and receiver stage becomes lower in high frequency bands as in millimeter waves if the circulator is used, a signal of the transmitter stage having high output power is introduced into the receiver stage. Thus, there exists a problem in that it is difficult to detect a weak reflection signal since the reception sensitivity of the radar system is deteriorated. Further, there is a problem in that the noise figure of the receiver stage is degraded due to a large leakage signal of the transmitter stage. Furthermore, there is a problem in that the characteristics of the entire radar system are degraded since the low-noise amplifiers or mixers constituting the receiver stage are saturated.
Meanwhile, when the coupler is used, half of the transmission power is consumed across an end terminal resistor of the coupler since one port of the coupler ends up with a resistor having resistance of 50 ohms. Further, even through received, the other half of the power is introduced into the transmitter stage. Thus, power loss of 6 dB occurs theoretically. If a circuit operates in millimeter wave bands, it is difficult to obtain a high output since small-sized devices are used to increase its frequency characteristics. If the power loss of about 6 dB occurs, a problem is caused which adversely affects the performance of the radar system of millimeter wave bands as well as degrades the signal-to-noise ratio (SNR) characteristics of the radar system.
FIG. 1A and FIG. 1B are views for explaining drawbacks of a conventional radar system using the linearly polarized wave antenna. In FIG. 1A, if a signal reflected from a target object rotates during returning, signal attenuation occurs. That is, if a signal having a magnitude of E in the vertical direction rotates by as much as an angle θ in the right direction, the magnitude of a reflection signal received through the receiver stage is reduced to Ecosθ. Thus, the sensitivity of the radar system is degraded.
FIG. 1B shows that two opposite radar systems exist. Transmission signals between the radar systems serve as interference signals since the transmission signals are much larger than a signal reflected from a target object. Thus, there exists a problem of difficulty in receiving desired signals.